But there seem to be two different questions here:
1: How long before any data you have built up is deleted after the trial period expires?
2: How long after the trial period expires before your username is deleted?

In my case, I was able to re-activate my username about 5 or 6 months after the trial period. I didn't really have any data associated with the account so I can't remember if that was deleted. However, two dormant .mac email only accounts which I subsequently resurrected came back with all the data intact and they had languished much loger than 6 months.

I have no idea if there's a strict deadline that Apple enforce about all this - why not ask them?

MacRumors attracts a broad audience
of both consumers and professionals interested in
the latest technologies and products. We also boast an active community focused on
purchasing decisions and technical aspects of the iPhone, iPod, iPad, and Mac platforms.